Mineral oils very often receive the addition of commercial markers, either alone or mixed with dyes, in small quantities comprised generally from 10 ppm to 40 ppm, so as to characterize and diversify said oils, mainly for taxation reasons, depending on use. A same universal oil can in fact be subjected to different taxation rates when it is intended for different uses (e.g., automotive use, heating, industry). Use is therefore characterized by the presence or absence of dyes and/or markers. It is thus possible to contrast fraudulent uses by means of qualitative and quantitative testing of the dye and marker that is present in the oil. Generally, the presence of the marker is detected by extracting it with an acid or basic solution depending on the type of said marker. The extraction solution therefore assumes a specific color, so that the presence of the marker can be detected by means of a field test which can be performed even by a non-expert. Moreover, spectrophotometric analysis allows to determine precisely the type of marker by means of the wavelength of the absorption peak and the quantity of the marker by measuring the absorbance. In order to identify fraudulent blending, the minimum detectable concentration generally must be at least 10 times lower than the amount used. The best-known commercial markers include those disclosed in patents DE3627461 (in the name of BASF) and EP0509819 (in the name of MORTON).
DE3627461 discloses dyes with azoic aromatic para-amino structure, which can be extracted from the mineral oil to which they are added by means of an acid aqueous solution which assumes an intense red color. Among the molecules of DE3627461, the most successful one is C.I. (color index) Solvent Yellow 124 (known as Sudan Marker 455-BASF). The product, as well as the entire class of molecules to which it belongs, disclosed by DE3627461, is characterized in that it gives an intense yellow color (maximum of 412 nm in xylene) to the mineral oil to which it is added. However, this aspect is unwanted in many cases, since it entails for example altering the color of any blends of mineral oils which comprise the tagged oil.
EP0509818 discloses non-coloring markers having an azoic-phenolic structure, which do not color significantly the mineral oils to which they are added. The main representatives of the class disclosed in EP0509818 are Mortrace ST and Mortrace SB. The molecules of EP0509818 and the two Mortrace products, in particular, suffer the drawback of having a low spectrophotometric absorbance in the extractant (generally a ternary basic solution of water-amine-KOH), therefore requiring high doses in order to be detected.